I thought it was clear. The birds are angry because the pigs stole their eggs. The birds will continue to be angry as long as the pigs don't return the stolen eggs. Now why did the pigs steal the eggs? Only god knows.

As a chicken you should understand that one must not steal someone else's eggs.

I agree that stealing eggs is unacceptable, launching yourself causing destruction and harm isn't. I think the birds and pigs should negotiate with each other so the angry birds can get their eggs back.